The aflatoxins are a family of carcinogenic mycotoxins of polyketide origin. Aflatoxin B1 is the most potent liver carcinogen known for the laboratory rat. These toxins occur widely in many parts of the world as food contaminants and are an important environmental health hazard. The biosynthesis of this family of toxins is to be investigated using specific 13C/2H- and 14C/3H-labelled specimens of averufin, a probable early precursor of sterigmatocystin and aflatoxin. These experiments will establish unequivocally the fate of isotopically-labelled centers through the intermediates of the biosynthetic pathway by Ft-nmr or degradation. This information will impose restrictions on the possible mechanisms that can account for these biotransformations. Further studies are described which are intended to reveal the mechanistic details and stereochemistry of these processes.